The purified HIV proteins will be provided to Selectide for screening of peptide libraries and to NeXagen for selection of RNA ligands. The existing expression and purification protocols will be scaled-up. New protocols will be developed for labeling HIV proteins to be used for the positive identification of beads containing binding peptide. Free peptide and RNA ligands will be characterized for their ability to bind HIV proteins and to inhibit activities in enzyme assays. These include inhibition of dimerization and determination of binding constants towards HIV proteins from quantitative binding experiments. Based on the results from binding studies, the proteins will be provided to component 4 of the proposal for co-crystallization with the most interesting and appropriate ligands. Initially, this approach will be applied to the isolated RNase H and polymerase domains of HIV-1 reverse transcriptase. Analogous strategies will be pursued for other gag-pol proteins of HIV.